Lithium manganese iron phosphate positive electrode material and preparation method therefor

Abstract

The present invention relates to the technical field of lithium-ion batteries. Disclosed are a lithium manganese iron phosphate positive electrode material and a preparation method therefor. The preparation method comprises: uniformly mixing raw materials with a solvent to obtain a primary mixture, wherein the raw materials include a phosphorus-containing compound, a manganese-containing compound, an iron-containing compound, a lithium-containing compound, and a carbon-containing compound; performing primary pretreatment and primary sintering on the primary mixture to obtain a primary sintered material; uniformly mixing the primary sintered material, an additive, and a solvent to obtain a secondary mixture, wherein the additive contains the element phosphorus; performing secondary pretreatment and secondary sintering on the secondary mixture to obtain a secondary sintered material, wherein the secondary sintering enables the phosphorus in the additive to be distributed on the surface of particles of the secondary sintered material to form a phosphorus-rich shell layer; and performing post-treatment on the secondary sintered material to obtain the lithium manganese iron phosphate positive electrode material. The present invention can reduce the powder resistivity of the positive electrode material, so that the attenuation rate of a voltage platform in the cycling process is slowed down, and the cycle life of the lithium manganese iron phosphate positive electrode material is prolonged.

Description

A lithium iron manganese phosphate positive electrode material and preparation method thereof

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to a Chinese patent application filed with the Patent Office of China on October 23, 2023, with application number 202311375375.1 and application name “A lithium manganese iron phosphate positive electrode material and its preparation method”, the entire contents of which are incorporated by reference into this application. Technical Field

[0003] The present invention relates to the technical field of lithium-ion batteries, and in particular to a lithium manganese iron phosphate positive electrode material and a preparation method thereof. Background Art

[0004] At present, commercial lithium-ion battery cathode materials mainly include ternary (nickel cobalt manganese oxide), lithium manganese oxide, and lithium iron phosphate materials. Among them, ternary materials have the highest energy density, but because they contain rare metal elements such as nickel and cobalt, the materials are expensive and their safety is poor. In commercial applications, components to prevent thermal runaway need to be added at the system level, which further increases the cost. Compared with ternary materials, lithium manganese oxide materials have lower costs, but due to the dissolution of manganese during use, the Jan-Taylor effect leads to poor cycle life, and the safety of lithium manganese oxide materials is also poor. Lithium iron phosphate materials have significant safety and cost advantages among the above three materials. After years of development, the lithium iron phosphate industry has become increasingly mature in material design and manufacturing technology. Its actual application capacity is close to the theoretical capacity, and the powder compaction density is as high as 2.6g / cc. Its performance potential has been basically fully developed.

[0005] With the continuous development of the new energy industry and the continuous advancement of material technology, the industry's innovation and progress are still continuing in an environment where the above three material systems have become increasingly mature. Among them, a variety of new positive electrode materials have been proposed, developed and applied, including high-voltage spinel nickel-manganese binary materials, lithium-rich manganese-based materials, and lithium iron manganese phosphate materials. Among them, lithium iron manganese phosphate materials have similar safety and capacity to lithium iron phosphate, and their voltage platform is about 0.6V higher than that of lithium iron phosphate. The mass energy density is 10-20% higher than that of lithium iron phosphate materials, and it has a significant energy density advantage. Therefore, lithium iron manganese phosphate materials are regarded as an upgraded version of lithium iron phosphate materials, with significant cost advantages per watt-hour.

[0006] However, there are still many problems in the performance of lithium manganese iron phosphate materials. For example, manganese dissolution during the cycle leads to rapid voltage platform decay, poor cycle life, and high powder resistivity, which hinder the large-scale commercial application of lithium manganese iron phosphate materials.

[0007] Summary of the Invention

[0008] The object of the present invention is to provide a lithium manganese iron phosphate positive electrode material and a preparation method thereof, which can reduce the dissolution rate of manganese during battery cycling.

[0009] To achieve the above object, a technical solution adopted by the present invention is to provide a method for preparing a lithium manganese iron phosphate positive electrode material, comprising:

[0010] uniformly mixing raw materials with a solvent to obtain a primary mixture, wherein the raw materials include a phosphorus-containing compound, a manganese-containing compound, an iron-containing compound, a lithium-containing compound, and a carbon-containing compound;

[0011] The primary mixed material is subjected to a pre-treatment and a sintering to obtain a primary sintered material;

[0012] uniformly mixing the primary sintering material, the additive and the solvent to obtain a secondary mixed material, wherein the additive contains phosphorus;

[0013] The secondary mixed material is subjected to secondary pretreatment and secondary sintering to obtain a secondary sintered material; the secondary sintering causes the phosphorus in the additive to be distributed on the particle surface of the secondary sintered material to form a phosphorus-rich shell layer;

[0014] The secondary sintered material is post-processed to obtain the lithium manganese iron phosphate positive electrode material.

[0015] The secondary pretreatment includes coarse grinding, fine grinding and drying; after the coarse grinding, the parameter D50 characterizing the slurry particle size is between 0.5 and 2.0 μm; after the fine grinding, the parameter D50 characterizing the slurry particle size is between 0.2 and 0.5 μm.

[0016] The diameter of the grinding beads used in the coarse grinding is 0.6-1.0 mm, and the diameter of the grinding beads used in the fine grinding is 0.2-0.35 mm.

[0017] The secondary sintering is specifically as follows: heating to a sintering temperature at a rate of 2-5°C / min and sintering for 3-15 hours, wherein the sintering temperature is between 500-850°C.

[0018] Wherein, the sintering temperature is between 600 and 700°C.

[0019] Wherein, the additive is at least one of phosphoric acid, monoammonium phosphate, ammonium dihydrogen phosphate, and lithium dihydrogen phosphate.

[0020] Wherein, the amount of phosphorus in the additive accounts for 0.05-0.5% of the total mass of the secondary sintering material.

[0021] The secondary mixture further includes a carbon-containing compound, and the secondary sintering forms a carbon coating layer on the particle surface of the secondary sintered material. The carbon-containing compound is at least one of glucose, sucrose, starch, and polyethylene glycol.

[0022] Wherein, the carbon coating layer is located on the outer surface of the phosphorus-rich shell layer.

[0023] In order to solve the above technical problems, another technical solution adopted by the present invention is: providing a lithium manganese iron phosphate positive electrode material prepared according to the preparation method described in the above technical solution.

[0024] The present invention has the beneficial effect of adding a phosphorus-containing additive during the secondary sintering process in the method for preparing a lithium iron manganese phosphate cathode material, compared to the prior art. This allows the phosphorus in the additive to be distributed on the surface of the secondary sintered material particles after the secondary sintering process, forming a phosphorus-rich shell. This phosphorus-rich shell can prevent manganese dissolution during the cycle, reducing the resistivity of the cathode material powder, thereby slowing the decay rate of the voltage platform during the cycle and improving the cycle life of the lithium iron manganese phosphate cathode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without inventive efforts. Among them:

[0026] FIG1 is a schematic flow chart of an embodiment of a method for preparing a lithium manganese iron phosphate positive electrode material according to the present invention;

[0027] FIG2 is a performance comparison of the embodiment and the comparative example regarding constant current charging ratio;

[0028] FIG3 is a performance comparison of the embodiment and the comparative example regarding capacity retention. DETAILED DESCRIPTION

[0029] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0030] Please refer to FIG1 , which is a schematic flow chart of an embodiment of a method for preparing a lithium manganese iron phosphate positive electrode material according to the present invention. The method comprises the following steps:

[0031] Step S11, mixing raw materials and solvent uniformly to obtain a primary mixture, wherein the raw materials include a phosphorus-containing compound, a manganese-containing compound, an iron-containing compound, a lithium-containing compound and a carbon-containing compound.

[0032] The raw materials for preparing lithium manganese iron phosphate are the raw materials commonly used in the prior art, phosphorus-containing compounds such as one or more of phosphoric acid, lithium dihydrogen phosphate, and ammonium dihydrogen phosphate, manganese-containing compounds such as one or more of manganese dioxide, manganese tetraoxide, and manganese carbonate, iron-containing compounds such as one or more of ferrous oxalate, ferric phosphate, and ferric oxide, lithium-containing compounds such as one or more of lithium dihydrogen phosphate, lithium carbonate, and lithium phosphate, and carbon-containing compounds such as one or more of glucose, sucrose, starch, and polyethylene glycol, wherein the carbon is mainly used to reduce iron ions in the iron-containing compounds to generate lithium manganese iron phosphate.

[0033] In order to improve the performance of positive electrode materials, oxides, carbonates, phosphates, metal organics, etc. of some elements are usually added to the raw materials to achieve element doping and maintain the material structure, such as Mg, Ti, V, Nb, W, Mo, Er, Co, Ni, Al and other elements.

[0034] The ratio of the various raw materials is also the ratio commonly used in the prior art, satisfying the element ratio of lithium manganese iron phosphate. Among them, water can be used as the solvent, and stirring or other methods can be used to promote the mixing of the raw materials and the solvent to obtain a uniform primary mixture.

[0035] Step S12: performing a pre-treatment and a sintering on the primary mixed material to obtain a primary sintered material.

[0036] After the raw materials and solvent are evenly mixed to form a primary mixture, it undergoes a pre-treatment process, specifically coarse grinding, fine grinding, and drying. The coarse grinding and fine grinding can be performed using methods such as sand milling. After grinding to an appropriate particle size, it is spray-dried to remove the solvent, and then subjected to a primary sintering process to obtain a primary sintered material. A chemical reaction occurs during the primary sintering process, and alkaline lithium carbonate residues are often left on the surface of the primary sintered material, which can adversely affect the performance of the subsequent positive electrode material.

[0037] Step S13: uniformly mix the primary sintering material, the additive and the solvent to obtain a secondary mixed material, wherein the additive contains phosphorus.

[0038] After obtaining the primary sintered material, it is evenly mixed with additives and solvents. The solvent can be water or other solvents that can dissolve and disperse the primary sintered material and the additives, wherein the additives contain phosphorus element P, to obtain a secondary mixed material.

[0039] Step S14 , performing secondary pretreatment and secondary sintering on the secondary mixed material to obtain a secondary sintered material; the secondary sintering allows the phosphorus in the additive to be distributed on the particle surface of the secondary sintered material to form a phosphorus-rich shell layer.

[0040] After obtaining the secondary mixture, it is first subjected to secondary pretreatment, which specifically includes coarse grinding, fine grinding and drying. The requirements for coarse grinding and fine grinding are that after coarse grinding, the parameter D50 characterizing the slurry particle size is between 0.5 and 2.0 μm, and after fine grinding, the parameter D50 characterizing the slurry particle size is between 0.2 and 0.5 μm. To achieve this indicator, it is preferred that the diameter of the grinding beads used for coarse grinding is 0.6 to 1.0 mm, and the diameter of the grinding beads used for fine grinding is 0.2 to 0.35 mm. During the above-mentioned coarse grinding and fine grinding process, the additive can also react with the residual alkali on the surface of the primary sintered material, thereby reducing the alkali content and improving the service life and safety performance of the subsequent positive electrode material.

[0041] The material is then subjected to a secondary sintering, specifically, the temperature is first raised to the sintering temperature at a rate of 2 to 5°C / min, for example, 2°C / min, 3°C / min, 4°C / min, 5°C / min, etc., and the sintering temperature is maintained at the sintering temperature for 3 to 15 hours. The sintering temperature is between 500 and 850°C, for example, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, etc., preferably between 600 and 700°C.

[0042] Among them, the above-mentioned additive is at least one of phosphoric acid, monoammonium phosphate, ammonium dihydrogen phosphate, and lithium dihydrogen phosphate, and the amount of phosphorus element in the additive accounts for 0.05-0.5% of the total mass of the secondary sintering material, for example, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.05%, etc.

[0043] During the secondary sintering process, the phosphorus element in the additive is enriched on the surface of the material particles, forming a surface phosphorus-rich shell, which has the effect of stabilizing the contact interface between the positive electrode material and the electrolyte.

[0044] Step S15: post-processing the secondary sintered material to obtain lithium manganese iron phosphate positive electrode material.

[0045] After obtaining the secondary sintered material, conventional post-processing operations such as air flow crushing and screening to remove iron are performed on it to obtain the lithium manganese iron phosphate positive electrode material.

[0046] By using the preparation method of the lithium manganese iron phosphate positive electrode material provided in this embodiment, the surface of the obtained positive electrode material particles has a phosphorus-rich shell layer. The phosphorus-rich shell layer can prevent manganese from being dissolved during the cycle, reduce the powder resistivity of the positive electrode material, thereby slowing down the decay rate of the voltage platform during the cycle, and improving the cycle life of the lithium manganese iron phosphate positive electrode material.

[0047] In some embodiments, the above-mentioned secondary mixture also includes carbon-containing compounds, such as glucose, sucrose, starch, polyethylene glycol, etc. The secondary sintering forms a carbon coating layer on the surface of the particles of the secondary sintered material. The carbon coating layer is located on the outer surface of the phosphorus-rich shell layer, further reducing the powder resistivity of the material and improving the cycle life of the lithium manganese iron phosphate positive electrode material.

[0048] Based on the same inventive concept, the present invention also provides a lithium manganese iron phosphate positive electrode material, which is prepared by the preparation method in the above embodiment. This embodiment has the same beneficial effects as the above embodiment, which will not be repeated here.

[0049] Based on the same inventive concept, the present invention also provides a lithium-ion battery, which includes the lithium iron manganese phosphate positive electrode material in the above-mentioned embodiment, and of course also includes a negative electrode material, a separator, an electrolyte, etc. that can be obtained from the scheme disclosed in the prior art. The lithium-ion battery also has the same beneficial effects as the above-mentioned embodiment, which will not be repeated here.

[0050] The performance improvement brought about by the technical solution provided by the present invention is illustrated below with reference to specific embodiments and comparative examples.

[0051] A positive electrode material was prepared according to the following steps S101-S105, which is recorded as Example 1:

[0052] In step S101, 17.9 kg of lithium carbonate, 66.9 kg of iron phosphate, 8.53 kg of glucose, 52 kg of manganese tetraoxide, 69.9 kg of lithium dihydrogen phosphate, 1.803 kg of magnesium carbonate, 0.144 kg of titanium dioxide, and 0.495 kg of niobium pentoxide are uniformly mixed with 220 kg of water to obtain a primary mixture.

[0053] In step S102, the primary mixture is introduced into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm), and the primary grinding particle size D50 = 1.3 μm; then introduced into a sand mill for fine grinding (zirconium bead diameter 0.3 mm), and the grinding particle size D50 = 0.35 μm; then spray drying is performed, and the spray drying air inlet temperature is 250°C and the air outlet temperature is 90°C; then sintering is performed under inert atmosphere protection conditions, raising the temperature to 650°C at a rate of 3°C / min, keeping warm for 7 hours, and finally cooling to obtain a primary sintered material.

[0054] Step S103 , 100 kg of the primary sintering material, 3.0 kg of glucose, 6.0 kg of polyethylene glycol (molecular weight 6000), 0.8 kg of lithium dihydrogen phosphate (additive) and 110 kg of water are mixed and stirred uniformly to obtain a secondary mixture.

[0055] In step S104, the secondary mixture is introduced into a sand mill for coarse grinding (zirconium bead diameter 0.6 mm) to a grinding particle size D50 = 0.8 μm; then introduced into a sand mill for fine grinding (zirconium bead diameter 0.3 mm) to a particle size D50 = 0.3 μm; then spray dried, with an air inlet temperature of 250°C and an air outlet temperature of 90°C; then sintered under inert atmosphere protection conditions, rising to 700°C at a rate of 3°C / min, keeping the temperature for 7 hours, and finally cooled to obtain a secondary sintered material.

[0056] In step S105 , the secondary sintered material is subjected to air flow pulverization and sieving to remove iron, thereby obtaining the lithium manganese iron phosphate positive electrode material of Example 1.

[0057] Example 2 differs from Example 1 in that, in step S103, the amount of the additive lithium dihydrogen phosphate is 1.2 kg. The other steps are the same, and the lithium iron manganese phosphate positive electrode material of Example 2 is finally obtained.

[0058] Example 3 differs from Example 1 in that, in step S103, the amount of the additive lithium dihydrogen phosphate is 1.6 kg. The other steps are the same, and the lithium iron manganese phosphate positive electrode material of Example 3 is finally obtained.

[0059] The comparative example differs from Example 1 in that, in step S103, the above-mentioned additive is not added, and the other steps are the same, and finally the lithium manganese iron phosphate positive electrode material of the comparative example is obtained.

[0060] Please refer to Table 1, which shows a comparison of performance data of the embodiments and comparative examples. The pH value, residual alkali content, powder compaction and resistivity of the positive electrode materials of Example 1, Example 2, Example 3 and the comparative example were respectively measured, and button batteries were made in the same manner. Then, the parameters characterizing the electrical performance (0.1C gram capacity, initial efficiency, 1.0C gram capacity) were respectively measured using the method disclosed in the prior art.

[0061] As can be seen from Table 1, with the increase in the amount of additives added, the residual alkali content and pH of the material are significantly reduced, and the gram capacity first increases and then decreases. In addition, the powder resistivity of the material is significantly improved, indicating that the surface of the positive electrode material particles prepared by the present invention has a phosphorus-rich shell layer, thereby reducing the powder resistivity of the positive electrode material.

[0062] Table 1 Comparison of performance data of examples and comparative examples

[0063] Further, please refer to Figures 2 and 3, which are performance characterizations of the constant current charge ratio and capacity retention rate, respectively. It can be seen that with the increase in the amount of additives added, the cycle life of the material is greatly improved, and the constant current charge ratio attenuation rate during the cycle is significantly reduced, indicating that the positive electrode material prepared by the present invention can prevent manganese dissolution during the cycle, thereby slowing down the decay rate of the voltage platform during the cycle and improving the cycle life of the lithium manganese iron phosphate positive electrode material.

[0064] The above description is only an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the contents of the description and drawings of the present invention, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate positive electrode material, characterized in that: include: The raw materials are mixed evenly with a solvent to obtain a primary mixture, wherein the raw materials include a phosphorus-containing compound, a manganese-containing compound, an iron-containing compound, a lithium-containing compound and a carbon-containing compound; The primary mixed material is subjected to a pre-treatment and a sintering to obtain a primary sintered material; The primary sintering material, additive and solvent are uniformly mixed to obtain a secondary mixed material, wherein the additive contains phosphorus element; The secondary mixed material is subjected to secondary pretreatment and secondary sintering to obtain a secondary sintered material; the secondary sintering allows the phosphorus in the additive to be distributed on the particle surface of the secondary sintered material to form a phosphorus-rich shell layer; The secondary sintered material is post-processed to obtain the lithium manganese iron phosphate positive electrode material.

2. The preparation method according to claim 1, characterized in that: The secondary pretreatment includes coarse grinding, fine grinding and drying; after the coarse grinding, the parameter D50 characterizing the slurry particle size is between 0.5 and 2.0 μm, and after the fine grinding, the parameter D50 characterizing the slurry particle size is between 0.2 and 0.5 μm.

3. The preparation method according to claim 2, characterized in that: The diameter of the grinding beads used in the coarse grinding is 0.6-1.0 mm, and the diameter of the grinding beads used in the fine grinding is 0.2-0.35 mm.

4. The preparation method according to claim 1, characterized in that: The secondary sintering is specifically as follows: The temperature is raised to the sintering temperature at a rate of 2 to 5°C / min, and sintered for 3 to 15 hours, wherein the sintering temperature is between 500 and 850°C.

5. The preparation method according to claim 4, characterized in that: The sintering temperature is between 600 and 700°C.

6. The preparation method according to claim 1, characterized in that: The additive is at least one of phosphoric acid, monoammonium phosphate, diammonium phosphate, and lithium dihydrogen phosphate.

7. The preparation method according to claim 6, characterized in that: The amount of phosphorus in the additive accounts for 0.05-0.5% of the total mass of the secondary sintering material.

8. The preparation method according to claim 1, characterized in that: The secondary mixed material also includes a carbon-containing compound. The secondary sintering forms a carbon coating layer on the particle surface of the secondary sintered material. The carbon-containing compound is at least one of glucose, sucrose, starch, and polyethylene glycol.

9. The preparation method according to claim 8, characterized in that: The carbon coating layer is located on the outer surface of the phosphorus-rich shell layer.

10. A lithium manganese iron phosphate positive electrode material prepared according to the preparation method according to any one of claims 1 to 9.

Images